Process for modifying the water permeability of a subterranean formation

ABSTRACT

A process for decreasing the water permeability of a subterranean rock formation is disclosed. The process involves the use of a water-soluble polymer in conjunction with an aluminum citrate preparation. The polymer can be injected into the formation first, followed by injection of the aluminum citrate preparation. Alternatively, an aqueous solution comprising both the polymer and the aluminum citrate preparation can be injected into the subterranean formation. Yet another process includes mixing a dry composition comprising the polymer and the aluminum citrate preparation, then dissolving this mixture in an aqueous solution which is then injected into the rock formation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 08/340,585 filed Nov. 16, 1994, now U.S. Pat. No. 5,559,263 which isincorporated herein in its entirety by reference.

FIELD OF THE INVENTION

This invention provides novel aluminum citrate preparations in dry andliquid form suitable for enhancing polymer cross-linking in secondaryand tertiary oil recovery.

BACKGROUND OF THE INVENTION

Aluminum citrate is useful for crosslinking polymeric materials insubterranean formations to reduce the permeability of the formations towater. In oil producing formations, aluminum citrate has been used as acrosslinker for several years to improve oil recovery beyond what isrecoverable by primary methods. Primary recovery involves production ofthe oil from the formation by natural driving forces such as gasexpansion. Primary recovery normally only produces about one-third ofthe oil in the reservoir, leaving two-thirds of the oil still in placein the formation. Another one-third of the oil in the formation may berecovered by secondary or tertiary oil recovery techniques, referred toherein as improved oil recovery methods.

In an oil-bearing subterranean formation, oil, water, and sometimes gas,coexist in porous formation rock. Secondary recovery often involvesinjecting water into the formation via injection wells, in order tomaintain pressure in the reservoir, and to drive the oil to producingwells. A disadvantage of using straight water to improve oil recovery isthat water tends to move into the more highly permeable regions of rock,which are the easiest flow paths, and bypass the lower permeable rock.This results in uneven recovery of oil in the formation, where oilrecovery is high in the highly permeable rock and the oil in the lesspermeable rock is left behind. Once the oil is recovered in the highlypermeable zones, the zones become "watered-out" and increasing water isproduced along with decreasing oil, making oil recovery uneconomic in ashort time.

Secondary or tertiary oil recovery may involve adding a water-solublepolymer to the water injected into the formation in order to increasethe viscosity of the water. This results in a more even flow of waterinto highly permeable and less permeable zones, and ultimately more oilrecovery. The polymer performance can be improved substantially bycrosslinking it after injection into the formation. Crosslinked polymerforms a gel in the formation rock in the highly permeable watered-outregions that selectively blocks these regions to additional flow ofwater. Because oil and water are immiscible, they occupy separate flowpaths in the formation pores, so water flow in the rock is blocked to amuch greater extent than oil flow with these water-soluble polymericgels. The final result is higher oil recovery from the reservoir andless water production and recycling.

A material that is often used to crosslink polymeric materials inimproved oil recovery operations is aluminum citrate. The trivalentaluminum metal acts as the active crosslinker for the polymer, and thecitrate complexes the aluminum so that it is released slowly in thepresence of the polymer after the solution is injected into thereservoir. A typical improved oil recovery operation may involveinjection of a blended solution of dissolved water-soluble polymer andaluminum citrate into a reservoir over a period of several months.

A second area where gels are useful is in grouting operations. Thiswould apply in certain construction projects where water encroachment isa problem. Grouts are injected into the subterranean formation torestrict the flow of water into the construction area. Grouts can alsobe used as a barrier to prohibit subterranean water movement in certainsituations. For example, water encroachment on a foundation from anearby pond might require use of a grout. Another use is inenvironmental remediation, where grouts can be used to temporarily blockmajor water flow paths between a hazardous waste site and a potablewater table.

When aluminum citrate was first used in the oilfield, according to U.S.Pat. No. 3,762,476 issued Oct. 2, 1973, aluminum sulfate hydrate andsodium citrate dihydrate were dry blended, then mixed with water at thedesired concentration and pumped into the formation. There were severaldisadvantages of this method. First, the injected crosslinker solutionhad a very low pH, on the order of 2, which was corrosive to oilfieldequipment. Second, the sulfate in the dry blend increased the tendencyof certain oilfield waters to form sulfate scales. Third, the citrateand aluminum were not in contact for a sufficient time or in sufficientconcentration for substantial chelation of the aluminum by the citrateto take place. As a result, aluminum release was more rapid, withpremature and inconsistent gels formed too close to the wellbore, andaluminum lost to the formation via adsorption.

Manufacture of a liquid solution of aluminum citrate starting withaluminum chloride and aluminum sulfate is disclosed in British PatentNo. 1,598,709. This liquid solution is unsuitable for improved oilrecovery because the aluminum:citrate molar ratio is about 5.2:1, whichresults in unchelated aluminum in an acidic solution, with a pH of about4 or less. The unchelated aluminum precipitates as the pH is increasedto about 6.5, which is more suited for improved oil recovery. Themaximum ratio of aluminum to citrate which allows for fully chelatedaluminum is about 2.2:1.

An improvement over the original method of providing aluminum citrate toimproved oil recovery projects was disclosed in U.S. Pat. No. 4,447,364.This method involves mixing a stable liquid aluminum citrate solution ofpH 5.5 to 7.5 starting with aluminum chloride and citric acid. A 34percent solution of aluminum chloride and a 50 percent solution ofcitric acid are blended such that the ratio of aluminum to citrate isfrom about 1.5:1 to 2:1. The pH of the acidic mixture is then adjustedupward with either ammonium hydroxide or sodium hydroxide. Anintermediate aluminum citrate solution has an aluminum concentration ofabout 1 percent to 3 percent by weight. The final solution has analuminum concentration of about 2.25 to about 3% by weight. The aluminumis fully chelated by the citrate. This is the current preferred solutionfoe oilfield use. In certain situations, the practical use of thisaluminum citrate solution is limited. Transportation of the liquidproduct to remote areas is expensive due to the relatively low activealuminum concentration. Ground transport is extremely expensive overlong distances because the liquid product must be hauled in a tanker,which is usually full one way and empty on the return trip. Because thefinal product is a liquid, it must be either stored indoors or in aheated tank in extremely cold areas, where the temperature drops belowabout -20° F. Therefore, desirable improvements over the currenttechnology include increasing the active aluminum concentration in thesolution and making a dry aluminum citrate product which can be moreeasily transported over long distances and stored under harshenvironmental circumstances.

An alternative method of mixing liquid aluminum citrate, starting withsodium aluminate, is disclosed in U.S. Pat. No. 4,601,340. The citratesource is either sodium citrate or citric acid. The concentration of thefinal product may be from 3 to 3.5 percent by weight of aluminum, butthe patent teaches that the aluminum concentration should be no greaterthan about 3 percent by weight.

Aluminum citrate compositions having a molar ratio of 1:1 aluminum tocitrate have been disclosed, but this ratio is too low for practical usein gels because the aluminum is too tightly bound by the citrate, andtherefore takes a long time to react with the polymer. U.S. Pat. Nos.3,200,136 and 2,327,815 discuss the preparation of very dilute solutionswith a 1:1 molar ratio of aluminum:citrate. The aluminum concentrations,which are much less than that provided by the current technology, aretoo low for practical use in subterranean formations. In U.S. Pat. No.3,200,136, example 5 discloses a solid aluminum citrate material with analuminum:citrate ratio of 2:1 which has a solution pH of about 3 at 10percent solution, which would give a substantially lower pH in solutionshaving aluminum concentrations of 3 percent or more by weight. A pH of 3is too low for practical use in gels. U.S. Pat. No. 2,327,815 disclosesan aluminum citrate salt which is a 100 percent basic complex and doesnot have a chloride component. A solid aluminum citrate material madefrom aluminum nitrate and citric acid is discussed by Feng, et al.,"Aluminum Citrate: Isolate and Structural Characterization of a StableTrinuclear Complex," Inorg. Chem. (1990) 29:408-411, but again, theactive aluminum concentration is too low for practical use.

Other liquid preparations comprising aluminum citrate in solution areknown. See, e.g., Gallet, J. -P. and Paris, R. A., "Etude Thermometriquede la Formation des Complexes du Fer(III), de L'Aluminium et duGallium," Anal. Chim. Acta (1967) 39:341-348; Weise, G. and Veith, J.A., "Komplexbidung Zwischen Zitronensaure und Aluminium," Z.Naturforsch. (1975) 30b:446-453; Karlik, S. J. et al., "Aluminum-27Nuclear Magnetic Resonance Study of Aluminum(III) Interactions withCarboxylate Ligands," Inorg. Chem. (1983) 22:525-529; Ohman, L. -O. andSjoberg, S., "Equilibrium and Structural Studies of Silicon(IV) andAluminum(III) in Aqueous Solution. Part 9. A Potentiometric Study ofMono- and Poly-nuclear Aluminum(III) Citrates," J. Chem. Soc. DaltonTrans. (1983) pp. 2513-2517; Mak, M. K. S. and Langford, C. H., "KineticAnalysis Applied to Aluminum Citrate Complexing," Inorg. Chim. Acta(1983) 70:237-246; Lopez-Quintela, M. A. et al., "Kinetics andThermodynamics of Complex Formation Between Aluminum(III) and CitricAcid in Aqueous Solution," J. Chem. Soc. Faraday Trans. I (1984)80:2313-2321; Gregor, J. E. and Powell, H. K. J., "Aluminum(III)-CitrateComplexes: a Potentiometric and ¹³ C N.M.R. Study," Aust. J. Chem.(1986) 39:1851-1864; Ohman, L. -O., "Equilibrium and Structural Studiesof Silicon(IV) and Aluminum(III) in Aqueous Solution. 17. Stable andMetastable Complexes in the System H⁺ -Al³⁺ -Citric Acid," Inorg. Chem.(1988) 27:2565-2570; Shioyama, T. K. and Little, R. A., U.S. Pat. No.4,560,783; and U.S. Pat. No. 4,601,340. However, to Applicant'sknowledge, no previous workers have been able to achieve stable aluminumcitrate solutions having greater than 3.0 weight percent aluminum.

Aluminum citrate solutions have been used in oil recovery processes asdescribed, e.g., in Mack, J., "Process Technology Improves OilRecovery," Oil & Gas J. (1979) pp.67-71; Parmeswar, R. and Willhite, S.,"A Study of the Reduction of Brine Permeability in Berea Sandstone Withthe Aluminum Citrate Process," SPE Reservoir Eng. (1988) pp. 959-966;and U.S. Pat. Nos. 3,762,476, 3,833,061, 3,952,806, 3,981,363,4,018,286, 4,039,029, 4,120,361, 4,413,680, 4,488,601, 4,498,539,4,526,231, 4,569,393, 4,579,176, 4,657,944, 4,644,193 and 5,151,615.

Aluminum citrate solutions have also been used in polymerizationprocesses (as described in U.S. Pat. Nos. 3,533,973 and 3,839,255) andin pharmaceutical and related products (as described in U.S. Pat. Nos.3,874,390, 3,924,642, 4,274,427, 4,645,662, 4,591,384, and 5,162,378).Miscellaneous industrial uses of aluminum citrate solutions includethose described in U.S. Pat. Nos. 3,898,186, 3,910,805, 3,964,255,4,116,931 and 4,612,175. Again, applicant is aware of no such solutionscontaining greater than 3.0 weight percent aluminum.

Recently a dry aluminum citrate material was developed by Haarman &Reimer, Elkhart, Ind., with two samples privately submitted to theinventor for evaluation. The first sample contained 10.99 percentaluminum and 1 percent chloride, with about 8 percent of the materialunaccounted for. The second sample contained 10.21 percent aluminum and0.3 percent chloride, with about 14 percent of the material unaccountedfor. The test results on these two dry aluminum citrate samples suggestthat both materials were made with a source of aluminum which isrelatively free of chloride. A source of aluminum which is in a chlorideform is desirable for making an aluminum citrate product, because thechloride, which will be present in the final product, is generally anon-scaling ion when mixed with water prior to subterranean injection.

U.S. Pat. Nos. 3,294,860, 4,888,136, 4,898,842, 3,674,726 and 5,019,401,and Japanese Patent No. 92065802 mention the existence of an aluminumcitrate product as a powder or salt; however, no methods for making sucha product are taught. No commercial source of a dry aluminum citrateproduct has been found by applicant herein. References to commercialavailability of such a product in the prior art appear to be erroneous.

All publications and patents referred to herein are incorporated byreference in their entirety.

SUMMARY OF THE INVENTION

An aluminum citrate preparation is provided in dry form having at leastabout 7 percent aluminum by weight, at least about 1.1 percent chlorideby weight, and a molar ratio of aluminum to citrate of between about0.5:1 and about 2.2:1. The preparation is suitable for use in secondaryand tertiary oil recovery and is substantially free of sulfate so as toavoid undesirable scaling. These preparations are also substantiallyfree of nitrate.

The term "aluminum citrate preparation" refers to a productpredominantly comprising aluminum and citrate.

Depending on the method of making the dry aluminum citrate preparationof this invention, it may have an aluminum:chloride molar ratio of atleast about 1:3 and up to about 2:1, preferably about 2:1. In apreferred embodiment, the aluminum citrate preparation of this inventionhas at least about 11 percent aluminum by weight.

A method for making a dry aluminum citrate preparation of this inventioncomprises:

a. providing a first aqueous solution comprising analuminum-chloride-containing complex wherein the aluminum:chloride molarratio is at least about 1:3;

b. mixing said first solution with a second solution comprising citricacid to form a composition wherein the molar ratio of aluminum:citrateis at most about 2.2:1;

c. stirring said composition of step b. continuously at a ratesufficient to keep the resultant slurry in motion;

d. adjusting the pH of said composition to between about 5.0 and about9.0;

e. drying said composition.

The drying step may be carried out by any means known to the art and ispreferably carried out using drum drying means.

This dry aluminum citrate preparation, when solubilized in water or anaqueous medium, will form an aqueous solution having a pH between about5.0 and about 9.0 at a concentration of up to about 3 percent by weightaluminum. Liquid aluminum citrate preparations of this invention may bemade by solubilizing dry aluminum citrate preparations of this inventionin aqueous media.

Liquid aluminum citrate preparations of this invention, more accuratelyreferred to as solutions comprising aluminum citrate complex, may alsobe prepared by the following method:

a. providing a first aqueous solution comprising analuminum-chloride-containing complex wherein the aluminum:chloride ratiois at least about 1:3;

b. mixing said first solution with a second solution comprising citricacid to form a composition having a molar ratio of aluminum:citrate atmost about 2.2:1;

c. stirring said composition of step b. continuously at a ratesufficient to keep the resultant slurry in motion;

d. adjusting the pH of said composition to between about 5.0 and about9.0.

The pH adjustment of step d. is preferably carried out with aconcentrated basic hydroxide solution of the formula MOH, where M is analkali metal or ammonium cation. The aluminum-chloride-containingcomplex is preferably provided in the form of aluminum chlorohydratesolution having a molar ratio of aluminum:chloride of about 1:2, aconcentration of up to about 50 percent by weight and an aluminumconcentration of up to about 12.5 percent by weight. Thealuminum-chloride-containing complex may also be provided in the form ofa mixture of aluminum chlorohydrate and polyaluminum chloride solution,said polyaluminum chloride solution having an activity of between about25 percent and about 34 percent by weight, and a molar ratio ofaluminum:chloride of at least about 1:3 aluminum:chloride.

Liquid aluminum citrate preparations of this invention have at leastabout 3.1 percent by weight aluminum, a pH between about 5.0 and about9.0, a molar ratio of aluminum to citrate at least about 0.5:1 and atmost about 2.2:1, and are essentially free of sulfate and nitrate. Themolar ratio of aluminum to chloride is between about 1:3 and about 2:1,preferably about 2:1. Preferably such preparations have at least about1.1 percent chloride by weight.

The dry aluminum citrate composition of this invention may be used incombination with a water-soluble polymer to form apermeability-modifying composition in dry form comprising a mixture ofsuch polymer capable of cross linking in the presence of water andaluminum ions, and aluminum citrate having at least about 7 percentaluminum by weight, at least about 1.1 percent chloride by weight, and amolar ratio of aluminum to citrate of between about 0.5:1 and about2.2:1 in an amount sufficient to enhance cross linking of said polymer.

Any polymer known to the art may be used; preferably the polymer isselected from the group consisting of polyacrylamide, partiallyhydrolyzed polyacrylamide, carboxymethylcellulose, polyvinyl alcohol,polystyrene sulfonates, polyvinylpyrrolidone, AMPS(2-acrylamide-2-methyl propane sulfonate), and combinations thereof.Preferably the polymer is partially hydrolyzed polyacrylamide.

A process for decreasing the water permeability of a subterraneanformation using a liquid aluminum citrate preparation of this inventioncomprises:

a. injecting into the formation an aqueous solution comprising awater-soluble polymer capable of cross linking in the presence of waterand aluminum ions;

b. subsequently injecting into the formation an aqueous solutioncomprising aluminum citrate having at least about 3.1 percent by weightaluminum, a pH between about 5.0 and about 9.0 and a molar ratio ofaluminum to citrate at most about 2.2:1 in an amount sufficient toenhance cross linking of said polymer.

The aqueous solution comprising a polymer of step a. preferablycomprises about 50 to about 20,000 ppm of a polymer selected from thegroup consisting of polyacrylamide, partially hydrolyzed polyacrylamide,carboxymethylcellulose, polyvinyl alcohol, polystyrene sulfonates,polyvinylpyrrolidone, AMPS (2-acrylamide-2-methyl propane sulfonate),and combinations thereof.

Alternatively, the aqueous solutions of polymer and aluminum citrate maybe mixed prior to injection into the formation. In this case, the methodcomprises injecting into the formation an aqueous solution comprising awater-soluble polymer capable of cross linking in the presence of waterand aluminum ions, an aluminum citrate solution having at least about3.1 percent aluminum by weight of solutions at a pH between about 5.0and about 9.0 and a molar ratio of aluminum to citrate at most about2.2:1, said aluminum being present in an amount sufficient to enhancecross linking of said polymer.

The aluminum citrate may be provided to the injection site in the formof a dry aluminum citrate preparation of this invention and solubilizedjust prior to injection, or may be provided in the form of a liquidaluminum citrate preparation of this invention.

When a dry aluminum citrate composition is used, one method of thisinvention for decreasing the water permeability of a subterraneanformation comprises:

a. mixing (a) a dry composition comprising a water-soluble polymercapable of cross linking in the presence of water and aluminum ions, and(b) a dry aluminum citrate preparation having at least about 7 percentaluminum by weight, at least about 1.1 percent chloride by weight, and amolar ratio of aluminum to citrate of between about 0.5:1 and about2.2:1 in an amount sufficient to enhance cross linking of said polymer,with (c) an aqueous solution;

b. injecting said aqueous solution into the formation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of this invention involves first making aconcentrated aluminum citrate solution, then drying the solution toincrease the activity and transportability if desired and finally usingthe product to form gels in a subterranean formation.

The liquid aluminum citrate solution is mixed according to the followingprocess:

a. Provide an aqueous solution of an aluminum-chloride complex having amolar ratio of aluminum to chloride greater than 1:3. A suitablematerial is a solution of aluminum chlorohydrate, which has a molarratio of aluminum to chloride of about 2:1 and an active aluminumconcentration of about 12.5 percent.

b. Add a sufficient quantity of an aqueous solution comprising citricacid such that the desired aluminum:citrate ratio is attained. Asolution of 50 percent citric acid is suitable. The molar ratio ofaluminum to citrate should be less than about 2.2:1.

c. Allow the resultant acidic slurry mixture of the above two componentsto agitate at a rate sufficient to keep the fine particles of the slurrymoving.

d. Adjust the pH of the acidic mixture to between about 5.0 and 9.0while stirring, preferably to about 6.9, by adding a strong basicsolution while agitating vigorously. Aqua ammonia, which providesammonium hydroxide at about 30 percent by weight, works well.

e. The final aluminum citrate solution will have an active aluminumconcentration greater than about 3.1 percent by weight and preferablygreater than about 3.5 to about 4.5 percent by weight.

If a more concentrated dry aluminum citrate material is needed in orderto simplify long-distance shipping or storage under difficultenvironmental circumstances, the liquid product may be dried by a numberof conventional methods, such as spray drying, drum drying,crystallization or any suitable evaporative technique. It is possible toincrease the aluminum concentration in the dry product to at least about11 percent to 16.5 percent by weight.

The aluminum citrate product can be used in either liquid or dry form tocrosslink polymer in an aqueous solution for use as a water-blocking gelin a subterranean formation. In the liquid form, the aluminum citratesolution is mixed with an aqueous solution of polymer, then the mixtureis injected into the formation. In the dry form, the aluminum citratemay generally be used in several ways. In one way, the aluminum citrateis dissolved in water, then mixed with an aqueous solution of thepolymer, then the mixture is injected into the formation. In another,the dry aluminum citrate is mixed with a dry water-soluble polymericmaterial to form a combination dry product, which is then dissolved andinjected into the formation. A third way to use the dry aluminum citrateproduct is to add it directly to the aqueous polymer solution, theninject the mixture into the formation.

This invention provides a new aluminum citrate product which is animprovement over current technology in the following specific areas:

1. The liquid product can be mixed at up to about 5.1 weight percentaluminum, which is much higher than the current technology limit of 3percent.

2. The product is substantially free of sulfate and nitrate and containsless chloride than prior products made using aluminum chloride startingmaterials, which increases the aluminum concentration.

3. The amount of basic hydroxide required for neutralization of a givenamount of aluminum citrate is up to two thirds less than that requiredin conventional technology.

4. The liquid product can be dried to obtain a concentration of at least7 percent by weight aluminum, and up to about 16.5 percent by weight,with a preferred concentration of about 13 percent by weight aluminum.

5. The dry product is simpler than liquid product to transport andstore, especially under difficult circumstances such as remote fieldlocations and extremely cold environments.

6. The blended dry aluminum citrate/polymer product is simpler to feedbecause no extra chemical pump is required to add the aluminum citrateto the aqueous polymer, as there is with a liquid aluminum citrateproduct.

The liquid aluminum citrate product is made by first blending an aqueoussolution of aluminum chlorohydrate with an aqueous solution of citricacid while maintaining sufficient agitation to keep the entire mixturemoving throughout the reaction vessel. The aluminum chloride solutioncan be any concentration up to about 50 percent by weight, preferably inthe range of about 20 to about 50 percent and most preferably about 37.5percent. The citric acid solution can be any concentration up to about60 percent, with about 50 percent being the preferred concentration. Themixture of aluminum chlorohydrate and citric acid is acidic with a pH ofabout 1.0 to about 2.0 if the preferred concentrations are used.Immediately after blending the two materials, some heat is released,causing the mixture to warm slightly. When the aluminum chlorohydratesolution and the citric acid are mixed together, the mixture isinitially clear, but within about 20 to 30 minutes it begins to developa precipitate and becomes a slurry full of fine white particles withinabout an hour.

The slurry must be kept moving at a rate sufficient to keep theparticles from settling. Also, the reaction vessel must be free ofstagnant regions where particles could build over time. If the slurry isallowed to stop moving and the particles do settle due to a power outageor breakdown of the agitator, the agitation should be resumed again assoon as possible. If the agitation is resumed within about 24 hours, theslurry can still be used to provide a clear final product. If the slurryremains still for much more than about 24 hours, the final product willbe cloudy, with cloudiness increasing proportional to the time theslurry is allowed to sit.

After the slurry has been allowed to agitate, the pH is adjusted upward.The pH can be adjusted upward any time after the aluminum chlorohydrateand citric acid are blended, but a higher quality final product will beobtained if the slurry is allowed to stir for about 1/2 hour to about 24hours, more preferably about 1 to about 6 hours and most preferablyabout 2 hours. Any strong base in aqueous solution can be used to adjustthe pH upward, including but not limited to ammonium hydroxide, sodiumhydroxide or potassium hydroxide. Ammonium hydroxide is preferredbecause it is a strong base and the ammonium cation has the lowestmolecular weight, with the final result being a final product which ismore concentrated in aluminum. Concentrated ammonium hydroxide in water(aqua ammonia) has a concentration of about 30 percent by weight. Theaqua ammonia is added to the acidic slurry while agitating until the pHof the solution reaches about 5.0 to about 9.0, preferably about 6.0 toabout 8.0 and most preferably about 6.7 to 6.9. During the pH increase,the slurry thickens considerably over a short pH range from about 3 to4; because of this, it is important to keep the solution moving withvigorous agitation during the pH increase. Depending on how long theslurry was allowed to stir before pH adjustment, it will thin and clearwhen the pH is increased above about 5 to 6. The final product iscrystal clear with no evidence of precipitate. As the solution nears thepH endpoint, the base addition should be slowed, and the final pHadjustment should be carried out over a period of about 1 to about 2hours. The pH may creep either up or down, depending on the desiredendpoint, over the following 24 hours, so a final pH adjustment about 24hours after mixing is also advisable.

The final liquid aluminum citrate product made by this invention is aclear, blond colored solution with an aluminum content of at least about3.1 to about 5.1 percent by weight, with a molar ratio of aluminum tocitrate of less than about 2.2:1, depending on the initialconcentrations of the aluminum chlorohydrate and citric acid solutions.The product is stable for several months on standing. Stability of theproduct is best if the pH is adjusted to at least about 6.5; if a lowerpH is used the product should be used or diluted soon after mixing. Theamount of time the acid slurry is allowed to agitate before pHadjustment also has an effect on shelf life. If the pH is adjustedimmediately after the aluminum chlorohydrate and citric acid solutionsare mixed, the final solution will be clear initially, but will begin todevelop cloudiness and thicken within about a week. The solution willbecome progressively thicker and cloudier over the next few weeks, untilit becomes a thick white paste. The thick white paste is soluble inwater to form a clear solution at concentrations of about 1 percent orless, but it is difficult to handle on a large scale unless specialequipment is employed. The only case in which immediate adjustment ofthe pH is advisable is when the intention is to dry the productimmediately; in this case the dry product has good solubility in waterat the low concentrations at which it will be used. If the intention isto mix a liquid product which may stand for several months before using,the slurry should be allowed to stir for at least about 15 minutes, morepreferably about 30 minutes to about an hour and most preferably about 2to 3 hours. The slurry can be left to stir for up to three weeks beforepH adjustment with a clear final solution obtained after pH adjustment.If the slurry is stirred for more than about 3 weeks the final productis slightly cloudy rather than clear.

The maximum aluminum concentration in the final liquid product is about5.1 weight percent, and this can only be obtained if the initial acidslurry is mixed using about 50 percent aluminum chlorohydrate and about50 percent citric acid, with a molar ratio of about 1.9:1 aluminum tocitrate. This solution is clear and stable for several months onstanding, but is difficult to make on a large scale because the slurryis extremely thick at its peak and it is difficult to maintain agitationsufficient to prevent the particles from "hanging up" on the sides ofthe reaction vessel. When the particles do hang up they tend to build upand it is difficult to get them moving again. The final result when thishappens is a cloudy final product. The main use for this highlyconcentrated material is to make product intended for drying, tominimize the energy needed to dry the product.

The minimum aluminum concentration in the final liquid product should beabout 3.1 percent, which can be obtained by diluting the aluminumchlorohydrate to a concentration of about 20 percent by weight beforemixing with about 50 percent citric acid. Dilution of the aluminumchlorohydrate results in less severe thickening of the slurry during thepH adjustment, so agitation requirements are not as rigorous. However,at this low aluminum concentration the energy required for drying isincreased, so this is not recommended for products intended for drying.

The preferred maximum aluminum concentration in the final aluminumcitrate product is about 4.6 percent by weight. This is obtained bydiluting the aluminum chlorohydrate to about 37.5 percent with waterbefore mixing with the 50 percent citric acid. In this case, the slurrystage during pH adjustment does not thicken to a degree such thatagitation is extremely difficult. This is therefore an optimum finalaluminum concentration to work with. The solution is stable for severalmonths on standing as a liquid, and can be dried with less energyrequired relative to the 3.1 percent aluminum solution.

When aluminum chlorohydrate is used to make the aluminum citrateproduct, the chloride concentration is minimized, because the molarratio of aluminum to chloride in aluminum chlorohydrate is about 2:1.With current technology, this is the highest ratio of aluminum tochloride available in an aqueous solution. While the chloride isconsidered benign in the final product in terms of interfering withproduct efficacy or causing scale formation, it is desirable to minimizethe chloride concentration so that the aluminum citrate concentration inthe final product is maximized. However, there may also be cases whereother sources of aluminum, such as polyaluminum chloride, or aluminumchloride are more readily available or sufficiently less costly thanaluminum chlorohydrate to justify their use. The current technologyinvolves the use of straight aluminum chloride, which has a molar ratioof aluminum to chloride of 1:3, so that much more chloride is present inthe final product. This can be improved by mixing aluminum chloride withaluminum chlorohydrate in any ratio in order to increase the molar ratioof aluminum to chloride and minimize the chloride concentration in thefinal product. In this case, the aluminum to chloride molar ratio in thefinal product will be in the range of about 1:3 to about 2:1, where thealuminum chlorohydrate is used to upgrade the aluminum chloride for thepurpose of increasing the aluminum concentration in the final product.After the two aluminum sources are mixed together, the citric acidsolution can be added to form the acidic slurry, then the pH can beincreased as described above.

If the aluminum citrate product is intended for shipment over extremelylong distances or for use and storage in extremely harsh environmentswhere temperatures frequently drop below freezing the product can bedried to simplify operations. The material dries easily and can be driedusing any common industrial drying technique. The Applicant has workedwith evaporation, crystallization and spray drying, and has foundevaporation via drum drying to work best.

Aluminum citrate can be evaporated at temperatures ranging from about35° C. to about 200° C., preferably in the range about 45° C. to about105° C. and most preferably at about 85° C. In the optimum temperaturerange of 45° C. to 105° C., the dried product is a white solid made upof a combination of translucent crystals, white crystals, and whiteamorphous powder. The product has a dry density similar to sodiumchloride salt. The product has a solubility of about 15 percent andredissolves readily to form a crystal-clear solution with a pH that isclose to the original pH of the liquid product prior to drying. Thealuminum concentration in the dry product depends on the aluminum sourcein the liquid product and the molar ratio of aluminum:citrate. Forexample, if the aluminum source is aluminum chlorohydrate and thealuminum:citrate ratio is 1.9:1, the aluminum concentration in the finaldried product is about 12.0-13.5 percent aluminum if dried in theoptimum temperature range. If the aluminum source is a blend of aluminumchlorohydrate and polyaluminum chloride, the aluminum content in thefinal dry product decreases as the polyaluminum chloride in the blend isincreased. If 100 percent polyaluminum chloride is used to make theliquid product, the aluminum concentration in the final dry product isabout 7 to 8 percent.

When the material is dried in the optimum temperature range, the finalproduct contains about 3-5 percent moisture. By drying at highertemperatures of about 150° C. to about 200° C. the product can be driedfurther in order to decrease the moisture and increase the aluminumcontent, to a maximum of about 16.5 percent. However, when the productis dried at this high temperature it may "burn," resulting in a darkenedproduct with limited solubility. Also, the product is more hygroscopicwhen the moisture content is driven below about 3-5 percent. At thelowest drying temperature of about 35° C., the product dries, but thetime required for complete drying is excessive and the final product isonly about 12.0 percent aluminum. The drying time decreases and thealuminum concentration increases as the drying temperature is increasedto about 75° C. to 85° C. At 85° C., the aluminum concentration in thedried product is about 13.5 percent. Beyond about 85° C. the aluminumconcentration does not increase further, so this temperature representsthe optimum temperature for drying.

On a large scale, an excellent means of drying the product is to use adrum dryer. The aluminum citrate dries readily on a drum dryer. In afield test using a double drum dryer at a temperature of 65° C., dryaluminum citrate product was produced at a rate of 250 pounds/hour.

Spray drying also has been pilot tested and has potential on a largescale, with optimization of the process to improve the consistency ofthe dried product. When aluminum citrate is spray dried, the finalproduct tends to have a light, fluffy consistency which is difficult totransport on a large scale because of the high volume taken up by thedried material. One way to remedy this problem would be to pelletize thespray dried material, but this would add to the cost of the finalproduct.

The aluminum citrate product in either dry or liquid form can be used tocrosslink water-soluble polymeric materials to form gels in subterraneanformations. Gels are commonly used in oilfield enhanced oil recoveryprocesses to selectively block areas of rock to further penetration bywater. A common practice is to first dissolve the polymeric material inwater using a chemical feed system designed to dissolve the polymer,then add the crosslinker using a positive displacement type chemicalpump to the dissolved polymer downstream, so that the polymer andcrosslinker are mixed in the water before the solution begins to movedown the wellbore and into the subterranean formation. Another commonpractice that has been described in U.S. Pat. Nos. 3,762,476, 3,833,061,3,952,806, 4,120,361, 4,488,601 and 4,498,539, is to inject theconcentrated aluminum citrate solution and the aqueous polymer solutionsseparately, so the polymer and aluminum are essentially separated untilthey have entered the subterranean formation and have had time to mix insitu. As with the first practice, the polymer is added to the waterusing a suitable polymer feed system, and the aluminum is added via achemical pump. The liquid aluminum citrate product described in thisinvention can be used as a crosslinker according to either of thesecommon practices. The dry aluminum citrate product is more versatile andcan be used in a number of ways, including but not limited to thefollowing three possibilities:

1) Mix the dry aluminum citrate with water to obtain a concentratedsolution, then inject the solution downstream of the polymer injection,according to common practice. This practice is known as continuous gelinjection.

2) Mix the dry aluminum citrate with water to form a concentratedsolution, then inject this solution separately, followed by separatepolymer solution injection according to common practice. This practiceis known as layered gel injection.

3) Blend the dry aluminum citrate with a suitable dry polymericmaterial. The blended dry product can then be added to the injectionwater using a suitable dry chemical feed system.

In the first two embodiments, the dry aluminum citrate is simply mixedwith water to obtain a concentrated aqueous solution, then the aqueoussolution is used according to common practice. The third embodiment isunique and can only be practiced where both the aluminum citrate productand the polymeric material are dry products and are water soluble. Thedry blend provides a very convenient means of using crosslinked polymer,because the single blended product can be injected using only a suitabledry chemical feed system without the need for an addition chemical pumpand liquid storage facilities downstream of the polymer feed system.Thus, a project operator saves up-front capital costs when preparing asite for gel injection.

There are many water-soluble polymeric materials that can be used forsubterranean gels, including, but not limited to polyacrylamide,partially hydrolyzed polyacrylamide, carboxymethylcellulose, polyvinylalcohol, polystyrene sulfonates, polyvinylpyrrolidone, AMPS(2-acrylamide-2-methyl propane sulfonate), or any combination of thesematerials. The most common embodiments use partially hydrolyzedpolyacrylamide (HPAM) and HPAM/AMPS copolymers. The polymer should havea high molecular weight, of about 1 to about 30 million, more preferablyabout 10 to about 30 million and most preferably about 20 to about 30million. When a higher molecular weight polymer is used, a lower polymerconcentration is required to obtain a given gel strength. The polymermust have some negative charge in the form of hydrolysis groups; thecharge should be in the range of about 0.1 to about 50 percent, morepreferably about 5 to about 40 percent and most preferably about 10 toabout 30 percent. The negative charge on the polymer influences the rateof cross linking and, to an extent, the strength of the final gel. Ahigher negative charge leads to faster crosslinking and a stronger finalgel.

Polymer concentrations can range from about 50 to about 20,000 ppm,preferably from 100 to 3000 ppm and most preferably from about 150 toabout 1200 ppm. At the extreme low end of the polymer concentrationrange, gel formation is very slow, allowing the gellable polymer mixtureto be injected well into the subterranean formation before gelformation; however, the gels are extremely weak and only work well informations with relatively low permeability, on the order of 10-50 md.At the extreme high end of the concentration range, strong gels tend toform relatively quickly, so the gels should only be used in cases wheredeep penetration is not needed or desired. This may apply in cases whereit is desired to block fracture systems close to either an injection ora producing wellbore. In the optimum concentration range of about 150 toabout 1200 ppm polymer, colloidal dispersion gels are formed which canbe placed deep in a subterranean formation where they can be used toblock large areas of highly permeable rock to the flow of water. Thepolymer concentrations in this range allow gel formation rates which aresufficiently slow to place the gel deep in the formation, yet the gelsare strong enough to block highly permeable areas of rock to the flow ofwater. Over the optimum concentration range, lower polymerconcentrations, on the order of about 150 to about 600 ppm, work well insituations where a relatively fresh water (<3 percent by weight totaldissolved solids) is available for injection and the averagepermeability of the affected subterranean formation is less than about1000 md. Higher polymer concentrations are needed in situations wherethe injection supply water has a relatively high total dissolved solidscontent (>3 percent by weight total dissolved solids) or where thepermeability of the affected subterranean formation is greater thanabout 1000 md.

The aluminum concentration in the gel depends on the polymerconcentration, water makeup and gel strength and formation time desired.Because aluminum concentrations depend on polymer concentrations,aluminum concentrations are often expressed as polymer:aluminum ratios.In the gel process described herein, the polymer:aluminum ratio canrange from about 1:1 to about 1000:1. The lower ratios in the range ofabout 1:1 to about 1:10 are best used in relatively high salinitybrines, with total dissolved solids concentrations greater than about 3percent by weight; in this type of brine the additional aluminum citratecan better compete with the ions present in the brine for gel reactionsites on the polymer molecules. Higher polymer:aluminum ratios of about1:10 to about 1:1000, more preferably about 1:10 to about 1:200, andmost preferably about 1:10 to about 1:100 can, be used when theinjection supply water is relatively fresh, with less than about 3percent total dissolved solids.

Gels comprised of a polymeric material and the aluminum citrate productdescribed herein either in liquid or dry form can be used in a varietyof processes where blockage of highly permeable regions of subterraneanformations are needed. In enhanced oil recovery, the gels can beinjected following injection of an aqueous solution of cationicpolyacrylamide. In this process, the cationic polyacrylamide provides alayer of cationic sites on the rock surfaces, which are generallyanionic, and the subsequent gel adheres to the cationic sites creating avery strong blockage in the affected rock. The gel can also be injectedfollowing injection of a strong potassium hydroxide (KOH) solution,which is used to stabilize clays in subterranean formations as describedin U.S. Pat. No. 4,280,560. In this case, the entire process involvesinjection of a potassium chloride (KCl) spacer, followed by KOH,followed by another spacer to move unreacted KOH out of the area,followed by the polymeric/aluminum citrate gel. The gel blocks the mostpermeable rock to further flow of water, allowing subsequent waterinjection to sweep residual oil out of the less permeable rock. Enhancedoil recovery processes commonly involve injection of aqueoussurface-active solutions designed to "wash" oil out of subterranean rockby lowering the interfacial tension between the oil and the injectionwater. Surface-active solutions may comprise alkaline agents,surfactants, combinations of surfactants and alkaline agents, andcombinations of surfactants, alkaline agents and polymeric materials.Polymeric/aluminum citrate gels can be injected before, between andafter injection of surface-active slugs to prevent further entry ofinjection solutions into highly permeable rock which has been washed outby the surface-active solutions. Another commonly employed enhanced oilrecovery process is imbibition, in which chemicals are injected topromote water entry into tight rock, so the water can better drive theoil out of these hard-to-reach places in the subterranean formation. Inthis case, the gels may be used to block the more permeable, washed outrock prior to the imbibition process. Polymeric/aluminum citrate gelscan also be used in conjunction with other gel process. In this case agel intended for use near an injection wellbore such as that describedin U.S. Pat. No. 4,683,949 might be used to divert the flow ofsubsequent polymer/aluminum citrate gel, which could then penetratedeeper into the formation.

In civil service, injection of polymeric aluminum citrate gels can beused in place of conventional grouting operations. Typically, groutingis used to prevent water encroachment into a civil service project, suchas constructing a tunnel below the water table. A problem with manytypes of grouts, such as cements and resins, is that penetration intothe subterranean strata is limited, and the relatively thin expanse ofgrout is subject to high stress, creating the need for frequent repairs.A polymeric/aluminum citrate gel can be injected much deeper into thestrata and can affect a relatively large volume of strata. The result isa longer-lasting blocking effect and little or no requirement forrepairs.

In the environmental remediation and cleanup field, there is a need forin-depth blockage of subterranean strata. A typical case might involvean abandoned mine site, where metals have leached into groundwater. Inthis case the gel can be injected into the permeable region ofsubterranean strata where it provides a barrier between the mine siteand the groundwater formation.

The methods of practicing the invention are illustrated by the followingexamples:

EXAMPLE 1

The following batch was mixed in the laboratory and illustrates a highconcentration of aluminum for a final liquid product with a molar ratioof aluminum to citrate of 1.9:1. To a 300 ml beaker, 60 grams of 50percent aluminum chlorohydrate solution was added, followed by 20 gramsof deionized water. 56.14 grams of 50 percent citric acid was added tothe dilute aluminum chlorohydrate all at once, then the solution wasstirred using a magnetic stirrer at a medium rate. The solution wasallowed to stir for 30 minutes. After about 15 minutes of stirring, thesolution began to develop turbidity, and after 30 minutes the solutionwas extremely cloudy with a very fine, white precipitate. At this point,aqua ammonia addition was commenced. The initial pH at the start of theaqua addition was about 1.30. As the aqua was added, the pH increasedand the slurry increased in cloudiness and thickness, peaking at a pH ofabout 3 to 4. As more aqua was added, the pH continued to increase andthe solution began to thin and decrease in turbidity, up to a pH of6.45, where it cleared completely. The pH was increased further to 6.90over the next hour, then the finished solution was stored. The totalaqua used was 30.86 ml. The aluminum concentration in the final aluminumcitrate product was 4.60 percent by weight.

EXAMPLE 2

The purpose of this example is to illustrate the effect of temperatureon drying of the aluminum citrate product. A liquid aluminum citratesolution was mixed, starting with 201.71 grams 50 percent aluminumchlorohydrate, 303.09 grams deionized water and 188.63 grams 50 percentcitric acid, in a 1000 ml beaker. The solution was allowed to stir for30 minutes using an overhead stirrer. A total of 104.6 ml aqua ammoniawas added as described in Example 1, to increase the pH of the mixtureto 6.9. The solution was then divided into 9 parts and dried at 9different temperatures. The results are summarized in the followingtable.

    ______________________________________                                                                Active                                                Drying         Moisture Aluminum in                                           Temperature    Removed  Dry Product,                                          °C.     %        %                                                     ______________________________________                                        45             74.0     12.4                                                  55             74.8     12.7                                                  65             75.1     12.8                                                  72             75.3     13.0                                                  85             76.2     13.5                                                  95             75.6     13.1                                                  105            75.8     13.2                                                  145            78.5     13.7                                                  185            81.3     15.8                                                  ______________________________________                                    

The product that was dried at temperatures from 45° C. to 105° C. waswhite in color with a yellow tint. The dry particles were mostlygranular, with some amorphous dusty solid also present. It dissolvedreadily in water to form a clear solution at 15 percent by weight. Thetwo samples that were dried at the higher temperatures of 145° and 185°C. were dark brown and black, respectively. Both of these materials werelimited in solubility, with only about a 500 ppm solution possible.

EXAMPLE 3

The purpose of this example is to illustrate drying of an aluminumcitrate product made using previous technology, starting with aluminumchloride. The active aluminum concentration in this dried productrepresents the lower limit of the technology introduced in this patent.358.3 grams of 26.11 percent aluminum chloride was added to a 1000 mlbeaker, followed by 148.8 grams of 50 percent citric acid. The acidicsolution was allowed to stir for 1 hour, after which time it was stillclear, with no evidence of precipitate. After this time, aqua ammoniawas added to increase the pH. The solution went through a slurry stageas the aqua was added, then cleared at a pH of about 6.5. The pH wasraised to 6.8, then the solution was drum-dried at 80° C. The driedmaterial was a white, flaky material which crumbled easily and containeda lot of fine white dust. The aluminum concentration was 7.5 percent byweight, and the amount of moisture removed was 66.6 percent. Compared tothe product made by drying aluminum chlorohydrate in Example 2 at 85° C.the aluminum concentration in the dry material starting with aluminumchloride was about 5 percent less by weight.

EXAMPLE 4

This example illustrates the use of liquid aluminum citrate made fromaluminum chlorohydrate in colloidal dispersion gels. In the lab, asolution of 300 ppm partially hydrolyzed polyacrylamide was mixed in anaqueous solution of 0.5 percent KCl. The polymer was a copolymer ofacrylamide and sodium salt of carboxylic acid, with a viscosity averagemolecular weight of about 25 million and a carboxylate content of about25 percent. To each polymer solution was added liquid aluminum citratemade from aluminum chlorohydrate. The purpose of adding the aluminumcitrate to the polymer solutions was to form gels suitable for blockingthe flow of water in subterranean formations. To determine whether thegels were capable of blocking a porous media, the gels were tested byflowing under differential pressure through screen packs made of five100-mesh stainless steel screens stacked tightly on top of each other.The screen pack provides a simulated porous media. The flow test data isused to determine "transition pressures" for the gels. The transitionpressure of a gel is the pressure at which it will resist flow in thescreen pack and represents the relative strength of the gel; the higherthe transition pressure the stronger the gel. The following table showsthe gel compositions and transition pressures as of 1 week after mixingthe gels.

    ______________________________________                                                          Gels                                                        Aluminum Citrate Solutions   Transition                                       Aluminum Aluminum:Citrate                                                                             Aluminum Pressure,                                    Weight % Ratio          ppm      psi                                          ______________________________________                                        3.20     1.9:1          15       2.1                                          3.20     1.9:1          7.5      5.1                                          3.20     1.9:1          3.75     4.6                                          4.24     1.7:1          15       1.3                                          4.24     1.7:1          7.5      5.3                                          4.24     1.7:1          3.75     5.5                                          4.26     1.5:1          15       3.1                                          4.26     1.5:1          7.5      5.3                                          4.26     1.5:1          3.75     6.0                                          ______________________________________                                    

The data shows that gels with substantial strength will form when lowlevels of aluminum are added to the polymer solution as aluminumcitrate. The best gels consistently formed when the aluminumconcentrations ranged from 7.5 to 3.75 ppm, or when the polymer:aluminumratios were 40:1 to 80:1. The gels mixed with the higher aluminumconcentration of 15 ppm, with a polymer:aluminum ratio of 20:1, wereslightly weaker. This is probably due to over crosslinking of thepolymer by the aluminum, which results in the polymer constricting andtaking up less space in solution; thus, the resultant gel is weaker. Thedata also shows that, as of one week after forming, the gels crosslinkedwith aluminum citrate mixed at aluminum:citrate ratios ranging from1.9:1 to 1.5:1 are similar in strength.

EXAMPLE 5

The purpose of this example is to illustrate the use of dry aluminumcitrate made from aluminum chlorohydrate in making gels, and to show theeffect of aluminum:citrate molar ratios on gel reaction rates. In thisexperiment, the dried aluminum citrate products were added to water anddissolved to make solutions of 1000 ppm aluminum by weight. The dilutealuminum citrate solutions were added to solutions of 300 ppm partiallyhydrolyzed polyacrylamide in 0.5 percent KCl, as described in Example 4above, to make gels. The resultant gels were tested for transitionpressure at four time intervals after mixing, as in Example 4 above. Thefollowing table summarizes the experiment:

    ______________________________________                                        Dry Aluminum Citrate                                                                       Gel                                                                     Aluminum: Alumi-  Transition Pressure, psi                             Aluminum                                                                             Citrate   num     24          3     5                                  Weight %                                                                             Ratio     ppm     Hours 1 Week                                                                              Weeks Weeks                              ______________________________________                                        12.84  1.9:1     15      4.8   3.3   1.1   1.2                                12.84  1.9:1     7.5     5.0   5.2   4.6   4.5                                12.84  1.9:1     3.75    5.4   6.2   5.5   5.5                                12.32  1.7:1     15      5.4   2.1   1.0   1.0                                12.32  1.7:1     7.5     5.2   4.4   4.3   4.1                                12.32  1.7:1     3.75    5.0   6.2   6.0   5.0                                11.65  1.5:1     15      6.0   3.1   1.0   1.1                                11.65  1.5:1     7.5     4.6   5.1   4.1   4.0                                11.65  1.5:1     3.75    4.6   6.1   5.0   5.1                                ______________________________________                                    

The results show that the gels reached final transition pressures on theorder of 4.0 to 5.0 psi, and that the final strengths were essentiallyindependent of the aluminum:citrate ratio in the range tested. Also, apolymer:aluminum ratio of about 20:1 is too low, as the gels tend todeteriorate over time after forming; this is consistent with the resultsof Example 4.

EXAMPLE 6

The purpose of this example is to show how dry aluminum citrate can bedry blended with a dry polymeric material to form a single drycombination product which can be used to form gels. In the lab, fifteendry blends of aluminum citrate and partially hydrolyzed polyacrylamidewere mixed. Five different aluminum citrate solids were used, varying inaluminum:citrate ratio from 1:1 to 1.9:1. Each aluminum citrate solidwas used to mix three gels at three different polymer:aluminum ratios,20:1, 40:1 and 80:1. The polyacrylamide was the same as that used inExamples 4 and 5. To mix a gel, a sufficient amount of dry blend to givea polymer concentration of 300 ppm was added directly to a solution of0.5 percent KCl in water while stirring. The solution was allowed tostir overnight, then was stored until needed for quantitative testing.The gels were tested periodically for transition pressure as in Example5. The data is shown in the following table.

    ______________________________________                                                          Gel Transition Pressure,                                    Aluminum:                                                                             Polymer:  psi                                                         Citrate Aluminum  24             3      5                                     Ratio   Ratio     Hours    1 Week                                                                              Weeks  Weeks                                 ______________________________________                                        1.9:1   20:1      6.2      8.3   8.1    7.3                                   1.9:1   40:1      6.0      8.7   9.4    10.0                                  1.9:1   80:1      5.3      8.3   9.5    9.5                                   1.7:1   20:1      3.5      5.0   5.7    6.0                                   1.7:1   40:1      6.3      8.0   9.0    11.0                                  1.7:1   80:1      4.0      9.0   10.3   10.0                                  1.5:1   20:1      4.0      7.2   7.3    6.2                                   1.5:1   40:1      4.2      7.8   9.0    8.3                                   1.5:1   80:1      1.0      7.8   9.3    9.6                                   1.3:1   20:1      3.0      7.3   6.5    6.6                                   1.3:1   40:1      <1       8.9   9.3    8.6                                   1.3:1   80:1      <1       10.4  9.4    9.4                                   1.1:1   20:1      <1       8.0   7.4    7.3                                   1.1:1   40:1      <1       11.4  10.7   11.0                                  1.1:1   80:1      <1       10.7  11.8   12.3                                  ______________________________________                                    

The data shows that the dry formulations tested gave final transitionpressures ranging from 6 to 12.3 psi, when the polymer concentration was300 ppm. For most in-depth applications, this is an adequate range ofgel strengths to work with. To decrease or increase the strength,respectively, the polymer concentration can be decreased or increasedusing a given aluminum citrate formulation and polymer:aluminum ratio.As the aluminum:citrate ratio decreases, the gel formation ratedecreases, due to the chelating effect of the extra citrate relative tothe aluminum. The aluminum:citrate ratio does not significantly affectthe final gel strength, however. The best polymer:aluminum ratio to workwith is in the range 40:1 to 80:1 and greater, which is consistent withthe results of Examples 4 and 5.

EXAMPLE 7

The purpose of this example is to provide some baseline data on how longit takes to dry a liquid aluminum citrate material and the final productquality. 20,588 lbs of liquid aluminum citrate was dried on a doubledrum dryer in a large-scale pilot trial. The initial product was 34percent solids. Dry product was produced at a rate of about 250lbs/hour. Over a 28-hour period, 7000 lbs of dry product was produced.The final product contained about 5 percent moisture, 8 percentaluminum, 31 percent chloride and was water soluble. The pH of a 5percent solution of the dried aluminum citrate was about 6.5. Trial gelswith the redissolved dried aluminum citrate product were successful.

While particular embodiments of the invention have been described, itwill be understood that the invention is not limited thereto, since manymodifications can be made, and it is intended to include within theinvention any such modifications as fall within the scope of the claims.

I claim:
 1. A process for decreasing the water permeability of asubterranean formation comprising:a. injecting into the formation anaqueous solution comprising a water-soluble polymer capable of crosslinking in the presence of water and aluminum ions; b. subsequentlyinjecting into the formation an aqueous solution comprising aluminumcitrate having at least about 3.1 percent by weight aluminum, a pHbetween about 5.0 and about 9.0 and a molar ratio of aluminum to citrateat most about 2.2:1 in an amount sufficient to enhance cross linking ofsaid polymer.
 2. The process of claim 1 wherein said aqueous solutioncomprises about 50 to 20,000 ppm of a polymer selected from the groupconsisting of polyacrylamide, partially hydrolyzed polyacrylamide,carboxymethylcellulose, polyvinyl alcohol, polystyrene sulfonates,polyvinylpyrrolidone, AMPS (2-acrylamide-2-methyl propane sulfonate),and combinations thereof.
 3. A process for decreasing the waterpermeability of a subterranean formation comprising:a. mixing(i) anaqueous solution comprising a water-soluble polymer capable ofcross-linking in the presence of water and aluminum ions with (ii) anaqueous solution comprising aluminum citrate having at least about 3.1percent by weight aluminum, a pH between about 5.0 and about 9.0 and amolar ratio of aluminum to citrate at more about 2.2:1, said aluminumbeing present in an amount sufficient to enhance cross linking of saidpolymer, thereby forming an aqueous mixture; and b. injecting saidaqueous mixture into the formation.
 4. The process of claim 3 whereinsaid aqueous solution comprises about 50 to 20,000 ppm of a polymerselected from the group consisting of polyacrylamide, partiallyhydrolyzed polyacrylamide, carboxymethylcellulose, polyvinyl alcohol,polystyrene sulfonates, polyvinylpyrrolidone, AMPS(2-acrylamide-2-methyl propane sulfonate), and combinations thereof. 5.A process for decreasing the water permeability of a subterraneanformation comprising:a. mixing (1) a dry composition comprising awater-soluble polymer capable of cross linking in the presence of waterand aluminum ions, and (2) a dry aluminum citrate preparation having atleast about 1.1 percent chloride by weight, and a molar ratio ofaluminum to citrate of between about 0.5:1 and about 2.2:1. saidaluminum citrate preparation capable of forming an aqueous solution at aconcentration of at least about 3.1 percent by weight aluminum, in anamount sufficient to enhance cross linking of said polymer, with (3) anaqueous solution; b. injecting said aqueous solution into the formation.6. The process of claim 5 wherein said aqueous solution comprises about50 to 20,000 ppm of a polymer selected from the group consisting ofpolyacrylamide, partially hydrolyzed polyacrylamide,carboxymethylcellulose, polyvinyl alcohol, polystyrene sulfonates,polyvinylpyrrolidone, AMPS (2-acrylamide-2-methyl propane sulfonate),and combinations thereof.